Translucent polycrystalline alumina (PCA) has been used in the lighting industry for many years as the material of choice to form the arc discharge vessels (also generally referred to as arc tubes) of high pressure sodium lamps (HPS) and ceramic metal halide lamps. Transparent sapphire has been used more sparingly as an arc tube material mainly because of its higher cost, but also because it is limited to regular geometric shapes like tubes, rods or sheets. Unlike PCA vessels which can be formed from powdered alumina into a wide variety of shapes from cylindrical to nearly spherical using a number of different manufacturing processes such as isostatic pressing, extrusion, slip casting, or injection molding, sapphire is typically grown slowly from a melt which limits the available shapes useable for discharge vessels to generally tubular. For these reasons and others, the use of sapphire has been limited to specialty lamps which require sapphire's transparency to meet stringent optical requirements, e.g., automobile headlamps.
Several attempts have been made to convert PCA to sapphire. For example, U.S. Pat. Nos. 5,549,746 and 5,451,553 describe a method of solid state crystal conversion (SSCC) that transforms MgO-doped PCA to sapphire at 200° C. below the melting point of sapphire. A Ga2O3 dopant also has been utilized to enhance the SSCC conversion. The enhancing effect was attributed to accelerated diffusion of Mg ions along grain boundaries due to the presence of the Ga ions. Similarly U.S. Pat. No. 6,475,942 discloses that Mo also enhances conversion. However, the use of Mo tends to darken the converted arc tubes which is not desirable for optical applications.
The conversion of MgO-doped PCA to sapphire by chemical addition with colloidal SiO2 has also been demonstrated. A local counter-doping of prefired tubes with SiO2 assisted the conversion without adversely affecting the sintering to translucency. The counter-doping caused abnormal grain growth to be triggered locally, and thereby controlled the initiation of the growth process. However, the reproducibility and the degree of conversion continue to be issues for the counter-doping conversion.